Pressure sensors or force sensors are used in numerous applications. Such sensors frequently are simultaneously subjected to different forces, namely forces from all three Cartesian directions x, y, z, shearing forces and moments. Exemplary applications are measurements of cutting forces in manufacturing processes, particularly milling operations, planing operations and the like, in which a translatory motion in the x-y direction is carried out by exerting a force. Depending on the respective application, several of these forces should be measured. In this context, forces in the z-direction, the x-direction and the y-direction, as well as the moment Mz in the z-direction, are usually of particular interest, wherein the surface of the workpiece to be processed extends in the x-y direction and its surface normal extends in the z-direction.
In such measurements, the individual force components need to be measured independently of the other forces or moments in any case. For this purpose, a sensor comprises a measuring body with several individual measuring elements that respectively measure one component. The measuring elements are usually realized in disks or perforated disks and consist of piezoelectric material. A certain number of positive and negative unit charges are emitted on opposite surfaces of the measuring elements proportionally to a force acting in a certain direction. Electrodes on the respective surfaces of the measuring elements collect these charges that ultimately are conveyed to charge converters or amplifiers in specially insulated lines. The measuring elements need to be installed in the sensor or in the measuring body under prestress such that negative forces can also be measured and contact with the electrodes is always ensured.
Different types of piezoelectric or piezoresistive plates can be used for measuring several force components or moments simultaneously. In a multicomponent sensor, for example, it is known to stack different plates on top of one another, wherein each plate can only measure one force component or one moment component and is relatively insensitive to all other effects of forces and moments. These plates are usually shaped in the form of disks or perforated disk. The advantage of plates in the shape of perforated disks can be seen in that a central mounting bolt can be used.
Particularly in piezoelectric measuring elements that are made of a crystal of symmetry group 32 and intended for measuring forces Fz acting perpendicularly upon the plate surfaces, however, it was determined that crosstalk occurs if transverse forces Fxy composed of the components Fx and Fy are applied orthogonally to Fz. When such a transverse force Fxy occurs, these Fz-measuring elements generate interference signals in that charges are emitted in certain quantities on the surfaces that are in electric contact with the electrodes.
A thusly created interference signal distorts the resulting measuring signal because it is superimposed on the signal generated due to the force Fz only. The interference signal can easily amount to 5 to 10% of the nominal value of the component Fxy that should not be measured by the measuring element. In this way, a systematic measuring error is created. During the measurement of Fz, the interference signal created due to crosstalk can be enormous in comparison with the signal that was caused by Fz and the amount of which should be measured by means of the measuring element, particularly if the amount of the transverse force Fxy is several times greater than the normal force Fz as it is the case, e.g., in milling or planing operations.
The interference signals generated in the direction Fz by the transverse force Fxy only occur in the aforementioned measuring elements that are cut from a piezoelectric crystal of symmetry group 32. The crystal plates are ultimately created by means of cuts of the crystal that were produced perpendicular to the crystallographic X-axis. This results in crystal plates with longitudinal effect that are particularly well suited for measuring forces, which is why they are preferably used in the aforementioned applications. In such measuring elements, however, the aforementioned crosstalk and the associated generation of interference signals occur in the aforementioned applications.
Other measuring elements with different crystallographic orientation behave differently when transverse forces Fxy act upon the measuring element orthogonally to the force Fz and therefore are not the object of the present invention.
These other measuring elements include, for example, different piezoelectric materials and materials from a different point group and measuring elements made of materials other than crystals such as, for example, of piezoresistive materials or piezoceramics. These other measuring elements also include all types of measuring elements that are designed for applications other than measuring a force acting perpendicular to the plate orientation such as, for example, for measuring an acceleration or a moment or a shearing force. These types of other measuring elements respectively are not the object of the present invention.